1. Field of the Invention
The subject invention relates to a bidirectional filter circuit having low-pass and high-pass filter channels connected between filter input and output to provide constant input/output impedance characteristics. The filter circuit of the invention has particular utility in applications requiring constant impedance characteristics, such as telephone lines, data transmission lines, alarm system lines, and the like.
2. Description of the Prior Art
The use of bidirectional filters generally having high and low-pass branches is known in the art. Zobel U.S. Pat. No. 1,615,252 discloses parallel connected high-pass and low-pass filters, with a repeater interposed in the high-pass branch. The Zobel patent relates to the use of filtering circuits in a transmission line and discloses the use of resistance networks as compensating networks.
Fry U.S. Pat. No. 1,559,864 relates to a filtering circuit having parallel connected low-pass and high-pass filter branches to give sharply defined boundaries to the transmitted bands or ranges of frequencies. Also note Herbert, Jr. U.S. Pat. No. 3,771,064 and Poschenrieder et al U.S. Pat. No. 3,794,936 which generally relate to high-pass and low-pass filter networks connected in parallel. The Herbert, Jr. patent particularly shows a bidirectional signal processing means for use in the television frequency range utilizing filter units and amplification means. The Whittle U.S. Pat. No. 2,041,098, Norton U.S. Pat. No. 2,076,248 and Bode U.S. Pat. No. 2,301,245 also disclose filter circuits having low-pass and high-pass filter branches connected between input and output.
Purington U.S. Pat. No. 2,762,018 discloses the utilization of reactance networks having structural symmetry. U.S. Pat. No. 2,260,494 to Terroni discloses designing filters having substantially constant impedance over a broad range and the use of compensating networks.